Steelmaking is often a batch procedure involving several steps. Molten metal is continuously produced in a blast furnace to produce molten iron, which is then transformed into steel by blowing oxygen. Steel scrap may be used to produce new steel in a primary melting vessel.
There is a broad variety of steel scrap, both in terms of composition (from plain carbon steel through to highly alloyed tool steel) and geometry (from finely shredded sheet through to large beams). The Electric Arc Furnace (EAF) is today the most common way to recycle steel from scrap. By melting the scrap in a furnace with an electrical current, new, functional steel can be produced from old products. Instead of deploying raw material resources, basic steel elements and valuable alloys can be reused, which is beneficial from both an economic and environmental point of view. After the scrap has been melted, the temperature is normally increased so that refining reactions can be carried out. Oxygen and carbon may be injected into the steel and slag phases respectively. However, the reactions can also create products which are detrimental to the steel quality and which therefore need to be handled carefully.
It is well known that the metallic oxides having a lower density float to the surface and generate what is known as a primary or natural slag. This slag is comprised mainly of oxides of iron, calcium, silicon, magnesium, manganese, and aluminum. The proportions of the non-metallic oxides is not compatible with the refractory system used to line the holding vessel, referred to as a ladle. Furnace slag in the ladle is known to increase production costs, and is known to contribute to poor alloy recovery, poor desulphurization and a general decrease in steel quality. It is a common practice to reduce the amount of primary slag as low as possible by mechanical methods including but not limited to skimming the slag, or through equipment design such as eccentric bottom tapping configurations.
However, slag is also important in steelmaking. In addition to absorbing impurities from the steel, the slag also protects the steel from the atmosphere. Furthermore, it protects the furnace and ladle walls from the electric arcs, thereby increasing electrical efficiency and improving refractory lining life by providing a coating on the working surface. It is therefore of great importance to maintain a high slag quality and provide it with foaming properties.
Slag is formed with the help of slag forming agents, such as lime, dolomite and/or fluorospar, or other common flux material. Slag properties such as, without limitation, viscosity, sulfur capacity, and phosphorus capacity vary with composition and temperature. Some of the metallic oxides that end up in the slag are acidic, so adding basic slag forming agents helps to keep the basicity of the slag at an appropriate level. High slag basicity (i.e. high lime to silica ratio) is also beneficial for phosphorus removal but care must be taken not to saturate the slag with lime as this will lead to an increase in slag viscosity, which will make the slag less effective.
Regardless of the effectiveness of the method employed to reduce primary slag, a certain amount of slag will remain, or be generated through the re-oxidation of iron or steelmaking alloys through contact with atmospheric oxygen. In order to prevent the re-oxidization, and to change the chemistry through dilution of any remaining primary slag, a synthetic slag is introduced. Synthetic slag is typically comprised mainly of calcined calcium oxide (quicklime), magnesia or dolomitic lime, and a fluxing additive to lower the melting point. The flux may be calcium fluoride, calcium aluminate or wollastonite. A de-oxidant such as aluminum or silicon bearing alloys may also be a constituent of the synthetic slag formulation to aid in reducing oxygen levels in both the slag and the resulting batch of steel. The act of reducing the oxygen content with aluminum or silicon is referred to as killing the steel or slag.
Synthetic slags are typically used in the form of powders or coarse granules. Powdered forms are typically less favorable due to dust formation, and the increased surface area of the particulate, which makes it more prone to re-hydration.
For economic as well as quality reasons, the synthetic slag does not facilitate the killing of the steel. Any aluminum or silicon contained in the synthetic slag formulation is present only to deoxidize the primary slag that is present. This is problematic, as the exact chemistry or mass of primary slag present can range quite dramatically. A well known alternative to aluminum or silicon slag de-oxidation is the utilization of calcium carbide.
Calcium carbide reacts chemically with the oxygen in the primary slag. It does not react with oxygen in the steel, and therefore does not alter the steel chemistry. The technical benefits of using calcium carbide as a slag deoxidant is well documented, however, such use is not widespread due to safety concerns regarding its storage and handling. Calcium carbide is very hygroscopic, and the result of any contact with water in any form is the formation of acetylene gas. Accordingly, there are regulations which require calcium carbide to be packaged only in enclosed metal containers. The packaging restrictions are such that calcium carbide as a part of a synthetic slag formulation is generally unacceptable for both economical and logistical reasons. Furthermore, the possible formation of acetylene gas and the resulting fire and explosion hazard has resulted in the general avoidance of using calcium carbide as a slag de-oxidant.